Such energy storage and power generation systems have been known for many years. Major limitations of these systems have resulted from the practical application of what seems to be a simple direct chemical process. Hazardous materials, efficiencies, system size, plugging and clogging, gas formation, "plating out" or precipitation of the materials, membrane diffusion limitations, cost of materials and cost of operation highlight the practical problems. Another limitation of such systems is the loss of power output as the system discharges.
The fundamental chemical process in these systems is characterized by a chemical equation where the action proceeds in one direction in the charging of the system and in the opposite direction during the power generation by the system. An example of a redox system is given by the following chemical equation, the term "redox" defining reactions in which a reduction and a complementary oxidation occur together. EQU Cr.sup.2+ +Fe.sup.3+ .revreaction.Cr.sup.3+ +Fe.sup.2+ Eq. 1
In this system, limitations exist since the chromium is expensive and the chromium and iron, meant to be on either side of a membrane, cross over contaminating the other side. This necessitates frequent reprocessing of the electrolyte. Furthermore, noble metal catalysts are required to promote the reaction. Also, the system pH must be controlled to prevent gas formation.
U.S. Pat. No. 4,485,154 discloses an electrically chargeable anionically active reduction-oxidation system using a sulfide/polysulfide reaction in one half of the cell and an iodine/polyiodide, chlorine/chloride or bromine/bromide reaction in the other half of the cell.
The overall chemical reaction involved, for example for the bromine/sulfide system is EQU Br.sub.2 +S.sup.2- .revreaction.2Br.sup.- +S Eq. 2
The electrochemical reaction takes place in separate but dependent bromine and sulfur reactions. The bromine reaction takes place on the +.sup.ve side of the membrane and the sulfur reaction on the -.sup.ve side of the membrane. When charging occurs the reaction goes from right to left and when discharging the reaction goes from left to right. During extended cycling of the cell ionic species diffuse through the membrane in an unwanted direction. Some sulfide diffuses into the +.sup.ve side of the cell and some of this sulfide is oxidised by the bromine in the +.sup.ve side to the sulfate SO.sub.4.sup.2-. Sulfates are not readily retrievable from the +ve electrolyte and thus represent a net loss of sulfur from the system.
U.S. patent application Ser. No. 128,117, now U.S. Pat. No. 5,496,659, discloses an electrochemical apparatus for energy storage and/or power delivery comprising multi-compartment cells with the +.sup.ve chamber and the -.sup.ve chamber of each cell being separated by at least one buffer chamber through which an idler electrolyte circulates. This apparatus is of particular use in carrying out the electrochemical reactions involved in the bromine/sulfide system discussed above. During extended cycling of the cell the S.sup.2- and/or HS.sup.- ions which otherwise would diffuse into the +.sup.ve chamber of the cell, migrate into the buffer chamber separating the +.sup.ve and -.sup.ve chambers of the cell. Some of the S.sup.2- and/or HS.sup.- ions which migrate into the buffer chamber from the -.sup.ve chamber of the cell are oxidised by bromine which migrates into the buffer chamber from the +.sup.ve chamber of the cell to form NaBr and sulfur as a precipitate. The free sulfur may be filtered from the idler electrolyte circulating through the buffer chamber and the filter periodically switched into the circulation of the electrolyte in the -.sup.ve chamber, the sulfur being restored by being resolubilized as polysulfide. However, the amount of S.sup.2- and/or HS.sup.- ions which migrate into the buffer chamber will generally be in excess of any bromine migrating into the buffer chamber. Thus over extended cycling there is a net loss of sulfur from the system.
We have now developed a process and apparatus which overcomes the above problem and prevents a net loss of sulfur from the -.sup.ve electrolyte of an energy storage and/or power delivery system in which the electrolyte circulating through the -.sup.ve chamber of the cell during power delivery contains sulfide.